In a synchronous direct-sequence code division multiple access (S-CDMA) system, users communicate simultaneously using the same frequency band via orthogonal modulation or spread spectrum. The number of orthogonal spreading codes (>1) limits the total capacity of the system. To increase the capacity of a CDMA system in a given service area, without requiring additional frequency bandwidth, space division multiple access (SDMA) can be employed.
Sectorization is the most common approach to providing space division multiple access (SDMA). Simply stated, sectorization splits the coverage area into multiple sectors, with each sector being serviced by independent antenna beams.
A SDMA/CDMA system employing sectorization has a user capacity that scales linearly with the number of sectors, but requires users along sector boundaries to be frequency isolated and, thus, additional frequency bandwidth is needed. That is, frequency division is typically employed along the adjacent sector boundaries, which requires additional frequency spectrum. Sectorization also suffers from decreased Erlang efficiency due to the limited capacity per sector. That is, the total supported traffic of a sectored cell is equal to the sum of the supported traffic in the individual sectors. However, the sum of the supported sector traffic is typically much less than could be supported in a cell with the same total number of channels, but with no sector constraints. As an example, a system with P=16 channels per sector and M=4 sectors can support 100 users per sector, or a total of 400 users per cell, assuming that GOS=0.02 and a traffic density of 0.1 Erlangs. A system with the same number of total channels (MP=64), but with no sector constraints, can support approximately 550 users under same assumptions. As such, since sectorization places restrictions on the locations of users, it is not the most efficient approach to supporting communications within a given area.
The concepts of SDMA and adaptive antennas has been actively researched. The use of SDMA and adaptive antenna systems in the context of CDMA has also been investigated. For example, reference can be had to “Smart Antennas for Wireless Communications: IS-95 & Third Generation CDMA Applications”, by J. C. Liberti and T. S. Rappaport, Prentice Hall, 1999. However, this text deals primarily with mobile CDMA applications, where smart antenna systems are used to provide isolation in asynchronous or quasi-synchronous CDMA systems. The text suggests nulling interfering users (interferers), but does not suggest that the interferers have the same CDMA spreading code for providing increased system capacity.
The use of antenna systems in conjunction with CDMA is also considered in U.S. Pat. No.: 4,901,307. In this patent antenna systems are used to point directive beams or to create interference patterns of “maximum signal to noise” at a given receiver location, and requires knowledge of the receiver location or the physical channel between the transmitter and the receiver. This patent also does not consider the S-CDMA case, nor the possibility of reusing the orthogonal spreading codes within the same coverage area.